1. Field of the Invention
This invention relates to methods of manufacturing an insulated conductor and the article produced thereby, and, more particularly, to methods of applying a plastic covering to an electrical conductor to provide a plastic covered conductor having a controlled adhesion of the plastic covering to the conductor.
2. Prior Art and Technical Consideration
Insulated electrical conductors, such as those employed in telephone installations, are often subjected to outdoor use or to conditions that expose the insulation to the deteriorating influences of light, weather, and possibly abrasion. With respect to telephone drop wire, which is the familiar black overhead wire comprised of two parallel spaced conductors that brings telephone service from the telephone pole to the home, it has been customary to enclose the conductors with an extruded rubber insulation covered by a cotton serving, and jacketed with a neoprene compound. Although such protective coatings have been in widespread use for many years and have proven satisfactory from most standpoints, there has been a long felt desire to develop an alternative, less expensive, insulation.
Of course, any alternative covering must have specific properties to fill the requirements of this type of wire. For example, it is important that the plastic covered drop wire be covered with an insulation material which has adequate properties to withstand exposure to the elements, as well as adequate low temperature flexibility, impact resistance, and abrasion resistance.
Each end portion of the wire is inserted into a metallic clamp. One of the clamps is attached to a subscriber's premises and the other one to a telephone pole prior to the electrical connection of the drop wire to wiring run inside the subscriber's premises and aerial distribution cables, respectively. At both the subscriber and the pole end of the drop wire, the retention of the wire in engagement with the clamp is effected by reactive forces exerted by the clamp on the insulation. If this is not transferred from the insulation to the conductors by the adhesion therebetween, the insulation may pull from the conductors and the entire weight of the drop wire would be held by the terminal connection. This may very well lead to a disconnection of the circuit.
On the other hand, if the adhesion between the composition and the conductors is too great, there may be problems in attempting to strip the covering from the conductors. Too great an adhesion could require an excessive scraping activity that would remove some of the metallic material from the surface portion of the conductors thereby increasing the electrical resistance and changing the conductivity thereof. Excessive scraping also unduly reduces the cross-sectional area of the wire or knicks it thereby reducing the strength properties of the wire to the detriment of its weight-supporting capability.
Drop wires have been manufactured in which coverings other than the three layer covering hereinbefore described have been used and which possess the requisite properties which includes optimum adhesion. These have generally involved the precoating of conductors with an adhesive followed by the final plastic insulation. This disadvantageously involves an extra manufacturing step.
A composition of matter for producing a plastic covered drop wire was disclosed and claimed in U.S. Pat. No. 3,579,608 issued on May 18, 1971 to John B. DeCoste. In that application, there was shown an abrasion-resistant insulating coating which included a plasticized polyvinyl chloride in combination with a brominated epoxy resin. It was disclosed that the composition possessed superior abrasion-resistant properties and adhered directly with the surface of a heated conductor. This avoided the necessity in prior art processes of applying a suitable adhesive to the conductor surface prior to the application of the insulation material thereover.
In the above-identified patent, it was disclosed that the conductors over which the plastic material is deposited should be preheated within the range of 200.degree. to 250.degree.C (392.degree. to 482.degree.F) prior to entering the cross head die of an extruder. It was disclosed in that patent that preheating the conductor to temperatures less than 200.degree.C resulted in weak bonding of the coating to the wire whereas temperatures in excess of over 250.degree.C adversely affected the properties of the plastic composition.
It has been found that in order to obtain an optimum permanent adhesion of the plastic composition to the conductor, certain changes to the process disclosed in the above-identified patent are necessary.